The hermetic seal of airplane indicator assemblies typically must be capable of constantly withstanding a shear force of at least one atmosphere (i.e. about 15 psi acts against the internal end faces of the housing when it is in a high altitude, low pressure environment) but it must also withstand temperatures from about -65.degree. F. to about +300.degree. F. It must also have strong resistance to solvents such as oil and gasoline as well as other destructive forces. In the past, the only method thought to satisfy these rigid requirements was a soft copper strip soldered to a joining seam of the split instrument housing assembly. However, several problems are presented by such prior practice.
For example, the copper strip and the assembly must undergo a pretinning process in order to quickly and uniformly solder the strip to the assembly housing (e.g. using SR 63 solder). Further, soldering of the copper strip requires high temperatures necessary for melting the solder. Thirdly, indicator assemblies typically have been and are made of aluminum housings (e.g. to conserve weight), however, aluminum typically cannot be directly soldered by conventional methods. Thus, aluminum housings typically must also be pre-plated with nickel in order to be properly soldered. The added process step of nickel plating is, of course, costly and time consuming.
A further typical difficulty in the process of conventional soldered copper band sealing of split instrument housing assemblies is the use of solder flux applied to the joints to be soldered. The flux is usually a corrosive threat to the electronic components within the housing and, of course, also requires an additional process step and, typically, a final cleaning step as well.
By use of such prior soldered copper band sealing methods, electronic components within the indicator assembly could not be repaired or removed without desoldering the copper strip through the application of high temperature heat--with the attendant opportunity to permanently damage the electronic components.
Thus, the presence of corrosive flux and the necessary application of high temperature heat presents a constant danger to reliability and life time of the electronic components contained in the indicator assemblies. Moreover, as mentioned, the soldering/desoldering method is in several respects costly and time consuming.
Although split housing indicator assemblies for housing electronic components in the aviation field typically have been hermetically sealed only through the use of soldered copper bands or strips, prior patents in other fields disclose methods of sealing, and sometimes hermetically sealing, other types of containers for use in other fields. One such method is disclosed in Kehe U.S. Pat. No. 3,403,810. The Kehe patent, however, teaches hermetic sealing of a plastic container of substantially ethylene copolymer. A closure skirt also comprised of copolymer is heat sealed to the copolymer container, thus hermetically sealing the container. The Kehe technique would also require the application of heat which, as already mentioned above, is a danger to electronic components within the indicator assembly.
C. E. Palmer in U.S. Pat. No. 3,426,936 discloses a tearstrip opener for sealed foil packages. Although it is there said that the strip of material may be affixed to the package by means of an adhesive, no such adhesive is disclosed in the Palmer patent that is believed suitable for sealing aircraft indicators. Nor is the possibility of sealing aircraft indicator housings with foil-backed acrylic transfer adhesive tearbands in any way suggested.
In Slomski U.S. Pat. No. 3,330,436, a seam release container uses a strip adhesively affixed and mechanically secured by an end tab. The Slomski patent is primarily directed towards an end tab that may be more easily pulled out thus releasing the seam and opening the container and which may be suitable for high speed automated production. As in the above-referenced patents, Slomski's disclosure is not directed to the problem of hermetically sealing aircraft indicator split housing assemblies.
In summary, tearbands and seam release containers per se are known in various arts. However, none of them, excepting the present invention, suggest use of a foil-backed acrylic transfer adhesive tearband to hermetically seal a split instrument housing assembly in aviation applications.
The present invention overcomes these problems and obstacles through the use of an acrylic transfer adhesive applied to a metallic foil band which, in turn, is used as a tearband device for structurally joining and hermetically sealing split aircraft instrument housing assemblies (e.g. containing indicators, electronic equipment and the like), without using the conventional high temperature soldered copper strip but which can nevertheless continuously withstand temperatures of up to 300.degree. F. and pressures up to one atmosphere.
Many advantages have been discovered in the use of such an acrylic transfer adhesive foil-backed tearband for this application. Such a tearband eliminates any need for solder pretinning; eliminates high solder temperatures; and eliminates any requirement for nickel plating and/or pretinning of aluminum housings. The present invention further eliminates the use of corrosive solder fluxes. Moreover, my acrylic transfer adhesive tearband can be easily and quickly installed and removed virtually an infinite number of times without damaging the nickel plated surface on aluminum housings (if present) or the aluminum housing itself. Cost savings in process and handling time of end item assemblies (and in the repair of same) are additional important features of the present invention.
It has been found that 3M brand A10 "ISOTACK" product No. Y-9469 acrylic transfer adhesive performs optimally having sufficient shear strength per unit area to permit reasonably sized tearbands designed to withstand 1 atmosphere or better of pressure differential (tending to separate the split housing) and to withstand temperatures of at least -65.degree. F. to +300.degree. F. Other acrylic transfer adhesives having substantially the same physical properties as the "ISOTACK" adhesive may also be employed.
The presently preferred commercially available acrylic transfer adhesive we believe is described more fully in U.S. Pat. No. Re. 24,906 to Erwin Uhlrich. The commercial product information literature includes the following:
A-10 "Isotac" adhesives may be used in general industrial applications where high bond strength, excellent temperature and solvent resistance, and outstanding shear strength properties are required.
1. Bond strength is dependent upon the amount of adhesive-to-surface contact developed. Firm application pressure develops better adhesive contact and thus improves bond strength.
2. To obtain optimum adhesion, the bonding surfaces must be clean, dry and well unified. Some typical surface cleaning solvents are isopropyl alcohol or heptane. Use proper procedures when handling solvents.
3. Ideal tape application temperature range is 70.degree. F. to 100.degree. F. (21.degree. C. to 38.degree. C.). Initial tape application to surfaces at temperatures below 50.degree. F. (10.degree. C.) is not recommended because the adhesive becomes too firm to adhere readily. However, once properly applied, low temperature holding is satisfactory.
4. Ultimate bond strength can be accelerated and increased by exposure of the bond to temperatures such as 150.degree. F. (66.degree. C.) for about 1 hour. Other heat ranges and time cycles may also be used to soften the adhesive. This provides better adhesive wetout onto the substrates.
A-10 "Isotac" acrylic adhesive is designed for applications requiring high peel and shear strength.
Also, if increased shear strength is desired, screws or shear pins or the like may be used to further resist shear forces caused by pressure differentials between the inside and outside of the hermetically sealed housing although such screws or pins are not an essential feature of the present invention.
Additionally, the metal used for the foil backing of the foil-backed acrylic transfer adhesive tearband may be the same or similar as the exterior metal surface of the split housing assembly so that both the tearband and the instrument assembly will have similar corrosive rates and/or other common physical characteristics. Presently, because of the aforementioned conventional process of soldering and the superior soldering characteristics of nickel, many assemblies in the field have housings comprised substantially of aluminum with nickel plating. For those assemblies, the metallic foil of the now described foil-backed acrylic transfer adhesive tearband should preferably comprise nickel. However, furture constructions of indicator assemblies may now eliminate the added nickel plating (since soldering will no longer be required) and, in such a case, the metallic foil of the foil-backed acrylic transfer adhesive tearband preferably comprises aluminum so as to have the same corrosive rate and/or other characteristics as the aluminum split housing assembly itself.
Importantly, the present invention may be used in current split housing assemblies for containing electronic indicator equipment.